According to the US Tire Manufacturers Association (USTMA), 35 percent of American drivers can't tell if their tires are bald. And since worn-out tires can greatly increase the likelihood of a crash, that's a recipe for disaster.
The USTMA says it's important to regularly inspect and maintain your tires. It recommends checking your tire pressures every month when the tires are cold (driving heats up your tires and makes the air inside expand). While doing this, you can also check each tire's tread depth. Below 2/32 inches and you'll want new tires. You don't need a ruler to check that, BTW -- you can easily test with a coin.